Boosted braking device with reduced travel

ABSTRACT

A boosted braking device for a motor vehicle having a master cylinder (200) and a pneumatic booster (100). The pneumatic booster (100) is capable of being operated by an operating rod (28) controlling the opening of a valve (26) in order to actuate a main hydraulic piston (34) in the master cylinder (200). The booster (100) has a rigid casing (10) divided in a leaktight fashion into first (12) and second (14) chambers by a moving partition (16) capable of driving a pneumatic piston (22) which can move relative to the casing (10). The main hydraulic piston (34) includes a hollow moving cylinder (38) having a secondary piston (40) which slides in a leaktight fashion in the interior thereof. The moving partition (16) is held fast to the moving cylinder (38) and is slidingly mounted on the pneumatic piston (22) so as to slide relative to the moving cylinder (38) in a forward direction over an axial distance (L) from an initial position of abutment against the rear of the pneumatic piston to a final position in which the moving cylinder (38) abuts the secondary piston (40). An annular piston (70) is held fast to the main hydraulic piston (34) when the travel of the annular piston (70) in bore (62,64) is less than the axial distance (L) while the annular piston (70) is capable of sliding over the main hydraulic piston (34) when the travel of the annular piston (70) in the bore (62,64) is greater than the axial distance (L) to obtain the maximum development of braking forces.

The present invention relates to a boosted braking device of the typeused for the braking of motor vehicles.

BACKGROUND OF THE INVENTION

Such braking devices have been known for a long time and conventionallycomprise, on the one hand, a master cylinder filled with a brake fluidand equipped with a main hydraulic piston designed to receive anactuation force composed of an input force and of a boost force, bothacting in an axial direction and, on the other hand, a pneumatic boostercapable of being operated through the application of the input force toan operating rod controlling the opening of a valve, in order to exertthe actuation force on the main hydraulic piston, the booster includinga rigid casing divided in leaktight fashion into two chambers by meansof a moving partition capable of being urged by a difference inpressure, between the two chambers, resulting from the opening of thevalve and of driving a pneumatic piston which can move relative to thecasing and which carries the valve, the input force being transmittedvia a reaction disk against which the pneumatic piston also bears inorder to supply at least some of the boost force to it.

A device of this type is well known in the prior art and is described,for example, in document U.S. Pat. No. 4,491,058.

For a long time it has been sought to improve these devices so that thetotal travel of the operating rod, and therefore the total travelavailable for the brake pedal, will be the result of a compromisebetween two contradictory parameters, so as to obtain what has becomeknown in the art as good pedal feel.

In fact, the travel of the operating rod has to be just sufficient forthe driver to be able to control the deceleration of the vehicle duringbraking in an optimum fashion. However, the total travel of theoperating rod is necessarily extended by a relatively significantinitial travel during which the hydraulic pressure in the brakingcircuit reaches a minimum value, beyond which any increase in pressurewill result in an effective braking action.

SUMMARY OF THE INVENTION

In order to solve this problem, a solution has already been proposed,for example in document FR-A-2,696,141, (U.S. Pat. No. 5,475,978)corresponding to the preamble of the main claim and according to whichthe main hydraulic piston of the master cylinder itself includes ahollow moving cylinder communicating with the master cylinder, whichreceives at least some of the boost force and inside which a secondaryhydraulic piston capable of receiving at least the input force slides,over an axial distance L, in a leaktight fashion and in the axialdirection, the moving partition being slidably mounted on the pneumaticpiston so as to be able to slide over the axial distance L relative toit in the direction of the master cylinder, from an initial relativeposition in which it is in abutment toward the rear against thepneumatic piston and bearing at least indirectly on the moving cylindertoward the front, when it is urged by a pressure difference.

Such a solution makes it possible to obtain the minimum braking pressurein response to a very small initial travel of the operating rod, whichinitial travel may even be imperceptible to the driver of the vehicle.

This solution does, however, involve an increase in the travel of themain piston of the master cylinder, and correspondingly an increase inthe axial length of the latter, and consequently an increase in thelength of the braking device, whose total overall size may prevent itfrom being fitted in the engine compartment of some vehicles.

The object of the present invention is therefore to provide a boostedbraking device whose effectiveness comes into play right at thebeginning of braking, that is to say after a very short initial travelof the brake pedal, and the overall size of which is not greater thanthat of a conventional braking device as illustrated, for example, bythe first above mentioned document.

To this end, the invention provides a boosted braking device of the typedefined above, in which an annular piston is held fast to the mainhydraulic piston when the travel of the latter in the bore is less thanthe axial distance L, the annular piston being capable of sliding overthe main hydraulic piston when the travel of the latter in the bore isgreater than the axial distance L.

In that way, the main hydraulic piston has a cross section which variesas a function of its position in the bore, this cross section beingappreciably greater over the first part of its travel. Such an enlargedcross section thus makes it possible to obtain the minimum brakingpressure desired after a travel of the piston corresponding to the axialdistance L.

Other objects, features and advantages of the invention will emergeclearly from the description which follows of one embodiment given byway of non-limiting indication with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in part section of a boosted braking device of the typeknown, for example, from the second above mentioned document;

FIG. 2 is a diagrammatic view in part section of a boosted brakingdevice in accordance with the present invention, represented in theposition of rest;

FIG. 3 is a view similar to that of FIG. 2, on a larger scale, thedevice being represented in a second position, and

FIG. 4 is a view similar to that of FIG. 3, the device being representedin a third position.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a view in part section of a boosted braking deviceconsisting of a pneumatic brake booster denoted overall by the reference100, and of a master cylinder, denoted overall by the reference 200.

The booster 100 is designed to be fastened in the usual way to adividing bulkhead between an engine compartment and the cabin of avehicle, and to be actuated by a brake pedal situated in this cabin. Themaster cylinder 200 controlling the hydraulic braking circuit of thevehicle is designed to be fastened to the booster 100.

By convention, that part of the braking device which points toward themaster cylinder 200 is termed the "front", and that part which pointstoward the brake pedal is termed the "rear". In the figures, the frontis thus to the left and the rear to the right.

The booster 100 itself comprises a casing 10 in the form of a shell ofaxis X-X', divided in leaktight fashion into a front chamber 12 and arear chamber 14 by a moving partition structure 16 comprising a rollingseal 18 and a rigid skirt 20, both connected to a pneumatic piston 22,the whole being able to move inside the casing 10 along the axis X-X'.

More precisely, the moving partition 16 is mounted, preferably via thecentral part 21 of its rigid skirt 20, so that it can slide freelyrelative to the pneumatic piston 22.

The front chamber 12 is permanently connected to a source (notrepresented) of partial vacuum via a non-return valve 24. The pressurein the rear chamber 14 is controlled by a valve 26, operated by anoperating rod 28 located along the axis X-X' and connected to a brakepedal 25 (FIG. 2).

The piston 22 is urged toward its rear position of rest, to the right,by a spring 30 bearing on the front face of the casing 10, and the rigidskirt 20 is itself urged toward its rear position of rest, to the right,by a spring 32 bearing on the piston 22.

The master cylinder 200 includes a main hydraulic piston 34 which iscomposite and which comprises, on the one hand, a moving and hollowcylinder 38 and, on the other hand, a secondary hydraulic piston 40.

The internal volume 42 of the moving cylinder 38 communicates with theinternal volume 36 of the master cylinder via at least one orifice suchas 44 made in the moving cylinder 38 in an axial direction.

Apart from the passage of fluid that these passages 44 between theinternal volume 36 of the master cylinder 200 and that of the movingcylinder 38 permit, this moving cylinder 38 slides in leaktight fashionin the master cylinder 200, sealing being obtained by virtue at least ofone annular collar 46.

The second hydraulic piston 40, for its part, slides inside the movingcylinder 38 in a leaktight fashion by virtue of an annular collar 48.Moreover, the moving cylinder 38 is held fast to the rigid skirt 20 bymeans of a connection piece 50.

The secondary hydraulic piston 40 constitutes the axial extension of apush rod 52 of the booster, capable of transmitting to it, on the onehand, the input force exerted on the operating rod 28 and, on the otherhand, a fraction of the boost force developed by the pneumatic piston22, these forces being transmitted, in a way known per se, via areaction disk 54 on one face of which this pneumatic piston and aplunger 56 actuated by the operating rod bear and the other face ofwhich bears on a bearing surface 58 held fast to the push rod 52.

Provision may be made for equipping the cylinder 36 with a limit stopfor limiting the free sliding of the secondary hydraulic piston 40relative to the hollow moving cylinder 38, and consequently limiting thefree sliding of the moving partition 16 relative to the pneumatic piston22. The same limitation may be obtained by means of limit stops providedon the central part 21 of the rigid skirt 20 and on the pneumatic piston22.

The operation of the device which has just been described is as follows.

When the operating rod 28 is in the position of rest represented in FIG.1, that is to say to the right, the valve 26 normally establishescommunication between the two chambers 12 and 14 of the booster.

As the rear chamber 14 is then subjected to the same partial vacuum asthe front chamber 12, the piston 22 and the rigid skirt 20 are pushedback to the right, into the position of rest, by the springs 30 and 32respectively.

Actuating the operating rod 28 toward the left has the effect, firstly,of displacing the valve 26 so that it isolates the chambers 12 and 14from each other then, secondly, of displacing this valve so that itopens the rear chamber 14 to atmospheric pressure.

The difference in pressures prevailing in the two chambers 12 and 14,then felt by the seal 18, exerts a thrust on the moving partition 20,which thrust tends to displace them forward. As the preload at rest onthe spring 32 is normally less than that on the spring 30, the movingpartition 20 moves by itself, compressing the spring 32.

The moving partition 20 in its movement drives the main hydraulic piston38 via the connection piece 50, thus causing the hydraulic pressure inthe internal volume 36 of the master cylinder 200 to increase, whichpressure rises sharply and becomes established, through the circulationof hydraulic fluid through the passages 44, in the internal volume 42 ofthe moving cylinder 38.

The movement of the moving partition 20 continues until it comes intoabutment on the pneumatic piston 22, or until the hollow cylinder 38comes into abutment on the secondary hydraulic piston 40. The pressureobtained in this operating phase corresponds to the minimum pressurerequired to initiate braking. This minimum pressure has thus beenobtained for a very small travel of the operating rod.

At this instant there is no longer any relative movement between theskirt 20 and the piston 22 which then move together when the pneumaticpiston 22 itself moves, that is to say if the driver of the vehicleincreases his force on the brake pedal.

The pressure in the internal volume 42 additionally causes there toappear, on the secondary hydraulic piston 40, a force which tends topush this secondary piston 40 back toward the operating rod 28, that isto say to the right in FIG. 1. This force on the secondary hydraulicpiston thus constitutes a reaction force which depends on the boostforce, and which opposes the force transmitted through the reaction disk54, allowing the boost force to be controlled by the input force bymeans of a compound reaction, that is to say one which is both hydraulicand mechanical.

Such a device therefore makes it possible to obtain a relatively highpressure in the master cylinder, owing to the fact that the skirt 20 andthe main hydraulic piston 34 of the master cylinder have covered atravel which is greater than that of the operating rod 28, some of thistravel of the piston 34 having been imperceptible to the driver. As aresult, the length of the master cylinder must therefore be such that itallows the main hydraulic piston 34 to cover this additional travel ofthe piston 34 relative to that of the operating rod 28 supplemented bythe travel necessary to obtain the desired effective braking effort andpressure.

The object of the present invention is precisely to avoid this drawback,and to allow the braking device to operate as has just been described,and therefore allow the main hydraulic piston 34 to cover an additionaltravel in order to obtain the minimum braking pressure without therebyappreciably increasing the total length of the master cylinder.

As represented in FIGS. 2 to 4, the bore in which the hollow cylinder 38slides is stepped, and includes a radial shoulder 60 between thesmaller-diameter front part 62 and the larger-diameter rear part 64.

The hollow moving cylinder 38 is itself stepped and includes a radialshoulder 66 between its front part 38a and rear part 38b, these partsinteracting respectively with the bore parts 62 and 64.

An annular seal 65 is located in the bore part 62 and is capable ofinteracting with the front part 38a of the cylinder 38 after apredetermined travel of the latter, as will be seen later.

An annular piston 70 is capable of sliding in leaktight fashion over thecylinder 38a by virtue of two seals 72 and 73, and in the bore 64 byvirtue of a seal 74. The annular piston 70, the bore 64, the shoulder 66and the cylinder 38a thus define a variable-volume annular chamber 76capable of communicating with the internal volume 42 of the movingcylinder 38 via at least one radial passage 78.

The cylinder 38 also includes a calibrated non-return valve 80 locatedbetween a passage 82 (FIGS. 3 and 4) for communication with the rearchamber 76, and a radial passage 84 emerging at the external surface ofthe part 38b of the cylinder 38, behind the annular collar 46.

Finally, a compression spring 86 is located in the chamber 76 betweenthe shoulder 66 of the cylinder 38 and the annular piston 70, in orderto urge the latter permanently forward. A limit stop 88 consisting, forexample, of a shoulder is formed on the part 38a of the cylinder 38 inorder to define the position of rest of the annular piston 70 relativeto the cylinder 38. A limit stop 90 is likewise formed inside thecylinder 38 in order to limit the backward movement of the secondaryhydraulic piston 40 relative to the cylinder 38.

As can be seen in FIG. 2, in the position of rest, the limit stop 88keeps the annular piston 80 at an axial distance L from the shoulder 60of the bore, and the limit stop 90 is situated at the same axialdistance L behind the secondary hydraulic piston 40. Furthermore, theannular seal 65 is at an axial distance L in front of the front end ofthe part 38a of the cylinder 38.

At rest, all the moving parts occupy their rear position represented inFIG. 2, except for the annular piston 70 which is bearing forward on thelimit stop 88. The internal volume 36 of the master cylinder, theinternal volume 42 of the moving cylinder 38, the volume 91 situatedbetween the annular piston 70 and the shoulder 60 of the annular chamber76 communicate with each other and with a low-pressure fluid reservoir(not represented) via an expansion orifice 92.

As seen earlier, actuating the brake pedal 25 causes the operating rod28 and the plunger 56 to move to the left. The valve 26 firstly isolatesthe chambers 12 and 14 from one another then, secondly, opens the fearchamber 14 to atmospheric pressure.

The difference in the pressures prevailing in the two chambers 12 and 14exerts on the moving partition 20 a thrust which tends to displace itforward by itself, the pneumatic piston 22 being kept in its rearposition under the effect of the preload of the spring 32.

The moving partition 20 in its movement drives the main hydraulic piston38 via the connection piece 50. As soon as the annular collar 46 hasgone past the expansion orifice 92, the hydraulic pressure in theinternal volume 36 of the master cylinder and internal volume 42 of thecylinder 38, in the volume 91 and in the annular chamber 76 starts torise.

As the non-return valve 80 has been calibrated not to open in thisoperating phase, the annular piston 70 accompanies the cylinder 38 inits movement under the action of the spring 86, so that the volume sweptby this cylinder 38/piston 70 moving gear is defined by the crosssection of the bore 64, decreased by the cross section of the secondaryhydraulic piston 40 which remains stationary during this operatingphase.

This movement continues over the entire axial distance L, as does theincrease in hydraulic pressure, until the annular piston 70 comes intoabutment against the shoulder 60, the limit stop 90 inside the cylinder38 almost simultaneously coming into contact with the secondaryhydraulic piston 40, depending on the manufacturing tolerances, justlike the front end of the part 38a of the cylinder 38 comes intointeraction with the annular seal 65.

The various elements of the master cylinder are then in the positionwhich has been represented in FIG. 3. The volume 91 is thereforeisolated from the rest of the master cylinder as it is contained by theseals 73, 74 and 65. Furthermore, the non-return valve 80 is calibratedto open at the pressure level reached at this moment in operation.

At this moment there can therefore no longer be any relative movementbetween the cylinder 38 and the secondary piston 40, or between theskirt 20 and the piston 22 which therefore move together when thepneumatic piston 22 itself moves, that is to say if the driver of thevehicle increases his force on the brake pedal.

In this situation, the cylinder 38 and the secondary piston 40 movetogether, so that the radial passage 78 goes past the annular seal 72while remaining behind the seal 73, thus closing off communicationbetween the annular chamber 76 and the internal volume 42 of the hollowcylinder 38, on the one hand, and the internal volume 36 of the mastercylinder on the other hand, shortly after the cylinder 38 has travelledthe axial distance L increased by the axial length of the seals 72 and65. Now being open, the valve 80 allows the fluid contained in theannular chamber 76 to return to the reservoir via the radial passage 84and the expansion orifice 92 or a compensation orifice 94 communicatingwith the low-pressure fluid reservoir, the spring 86 becomingprogressively compressed.

The pressure in the volumes 36 and 42 thus continues to rise, the volumeswept by this cylinder 38/secondary piston 40 moving gear being definedby the cross section of the front part 38a of the cylinder 38 sliding inthe annular piston 70, which remain stationary in this operating phase.Advantageously, provision may be made for this cross section to be equalto the cross section of the smaller-diameter bore part 62, thisprovision making it possible to use a tandem master cylinder asrepresented in FIG. 2.

As seen above, this pressure acting on the secondary hydraulic piston 40creates a reaction force which depends on the boost force and allows theboost force to be controlled by the input force, by means of a compoundreaction, that is to say one which is both hydraulic and mechanical.

The various elements of the master cylinder are therefore in theposition which has been represented in FIG. 4. The operation of themaster cylinder 200 and of the booster 100 is therefore identical tothat of a conventional boosted braking device.

When the driver of the vehicle releases his effort on the brake pedal inorder to terminate the braking action, all the moving elements arereturned to the rear. In particular, the non-return valve 80 closesagain so that the volume of the annular chamber 76 cannot increase. Theannular piston 70 therefore accompanies the cylinder 38 in its backwardmovement, until the annular collar 46 uncovers the expansion orifice 92.

At this moment, the front end of the part 38a of the cylinder 38 hasstopped interacting with the annular seal 65, so that communicationbetween the volume 91 and the rest of the master cylinder has beenreestablished and so that the spring 86 can push the annular piston 70back forward, to bear against the limit stop 88. All the elementstherefore resume their position of rest represented in FIG. 2, thusallowing another braking action as has just been described.

It can therefore clearly be seen that a boosted braking device has beenachieved in accordance with the given objective of the presentinvention. Indeed, in the first operating phase for obtaining theminimum effective braking pressure, during which the moving partition 16slides relative to the pneumatic piston 22, and in which the cylinder 38slides over the secondary hydraulic piston 70, that is to say for aninitial travel L of the moving partition 16, the volume of fluiddisplaced by the cylinder 38/piston 70 moving gear is appreciablygreater than the volume of fluid displaced by the cylinder 38/secondarypiston 40 moving gear during the second operating phase in which all theelements move together, except for the annular piston 70, in order toobtain the desired braking action and pressure.

Everything therefore happens as though the main hydraulic piston of themaster cylinder had a cross section varying along its travel in itsbore, its cross section being greater over the first part L of itstravel than over the second part, after it has covered the axialdistance L.

Thus, as a function of the features of the braking system of a type ofvehicle to which the boosted braking device of the invention is to befitted, and through a careful selection of the transverse sections ofthe annular piston 70 and of the front part 38a of the cylinder 38, ofthe axial distance L between the annular piston 70 and the shoulder 60and between the limit stop 90 and the secondary piston 40 at rest, andof the calibration of the non-return valve 80, it is indeed possibletherefore to obtain the minimum effective braking pressure withouthaving to extend the length of the master cylinder correspondingly, andwhile having a travel of the main hydraulic piston 34 equipped with theannular piston 70 greater than the travel of the operating rod 28.

Of course, the invention is not limited to the embodiments which havebeen described, but can in contrast receive numerous modifications whichwill be obvious to the person skilled in the art, without departing fromthe scope of the appended claims.

I claim:
 1. A booster braking device for a motor vehicle, comprising: amaster cylinder and a pneumatic booster, said master cylinder beingfilled with a brake fluid and equipped with a main hydraulic pistondesigned to receive an actuation force composed of an input force and ofa boost force, said input force and boost force acting in an axialdirection, said pneumatic booster being capable of operating byapplication of said input force being applied to an operating rodcontrolling an opening of a valve in order to exert said boost force onsaid main hydraulic piston, said booster having a rigid casing dividedin leaktight fashion into first and second chambers by means of a movingpartition capable of being urged by a difference in pressure betweensaid first and second chambers, said difference in pressure resultingfrom the opening of said valve, said booster having a pneumatic pistonwhich can move relative to the casing and carries said valve, saidpneumatic piston contributes at least to transmitting the boost force tosaid main hydraulic piston, said main hydraulic piston of the mastercylinder having a hollow moving cylinder with an internal volume andwhich slides in a bore and communicating with said master cylinder, saidhollow moving cylinder receives at least some of said boost force, asecondary hydraulic piston located in said hollow moving cylinderreceiving at least said input force slides over an axial distance inleaktight fashion and in said axial direction, said moving partitionbeing slidably mounted on the pneumatic piston so that it can slide overa relative axial distance in the direction of said master cylinder froman initial relative position in which said moving partition is inabutment against the rear of said pneumatic piston and bearing at leastindirectly on said moving cylinder toward the front in response to saidpressure difference, said device being characterized in that said boreis stepped and includes a radial shoulder between a smaller-diameterfront part and a larger-diameter rear part; said hollow moving cylinderis stepped and includes a radial shoulder between a smaller-diameterfront part and a larger-diameter rear part; an annular piston is heldfast to said main hydraulic piston when the travel of said hollow movingcylinder in said bore is less than said axial distance, said annularpiston is capable of sliding in leaktight fashion over said front partof said hollow moving cylinder and in said rear part of said steppedbore, said annular piston being capable of sliding over said mainhydraulic piston when the travel of said hollow moving cylinder in saidbore is greater than said axial distance, said annular piston, said rearpart of the stepped bore, said shoulder and said front part of saidhollow moving cylinder together define a variable-volume annularchamber, said variable-volume annular chamber being capable ofcommunicating with said internal volume of said hollow moving cylindervia at least one radial passage.
 2. The boosted braking device accordingto claim 1, characterized in that a communication path between saidvariable-volume annular chamber and said internal volume of said hollowmoving cylinder is interrupted shortly after said hollow moving cylinderhas moved said axial distance.
 3. The boosted braking device accordingto claim 2, characterized in that a calibrated non-return valve islocated in a communication passage between said annular chamber and alow-pressure fluid reservoir.
 4. The boosted braking device according toclaim 2, characterized in that a compression spring is located in saidannular chamber between said annular piston and said shoulder of saidhollow moving cylinder.
 5. The boosted braking device according to claim1, characterized in that, at rest, a first limit stop formed on saidfront part of said hollow moving cylinder keeps said annular piston atsaid axial distance from said shoulder of said bore.
 6. The boostedbraking device according to claim 5, characterized in that, a secondlimit stop formed in the hollow moving cylinder is situated at the axialdistance from a rear end of the secondary hydraulic piston.
 7. A boostedbraking device for a motor vehicle, comprising: a master cylinder filledwith a brake fluid and equipped with a main hydraulic piston designed toreceive an actuation force composed of an input force and of a boostforce, said input force and boost force acting in an axial direction;and a pneumatic booster capable of being operated by the application ofsaid input force to an operating rod for controlling the opening of avalve in order to exert said actuation force on said main hydraulicpiston, said booster including a rigid casing divided in leaktightfashion into first and second chambers by means of a moving partitioncapable of being urged in said axial direction by a difference inpressure between said first and second chambers resulting from actuationof said valve, said partition on moving driving a pneumatic pistonrelative to said casing, said pneumatic piston carrying said valve andcontributes at least to transmitting the boost force to said actuationforce, said main hydraulic piston of said master cylinder including ahollow moving cylinder which slides in a bore and communicating with themaster cylinder, said main hydraulic piston receiving at least some ofsaid boost force, a secondary hydraulic piston located inside of saidmain hydraulic piston capable of receiving at least the input force andwhich slides over an axial distance in leaktight fashion in said axialdirection, said moving partition being slidably mounted on saidpneumatic piston so as to slide over a relative axial distance in adirection of the master cylinder from an initial relative position inwhich said moving partition is in abutment toward the rear against thepneumatic piston and bearing at least indirectly on the moving cylindertoward the front when urged by a pressure difference; an annular pistonheld fast to the main hydraulic piston when the travel of the mainhydraulic piston in said bore is less than said axial distance, saidannular piston being capable of sliding over the main hydraulic pistonwhen the travel of said main hydraulic piston in said bore is greaterthan said axial distance, said bore being stepped and including a radialshoulder between a smaller diameter front part and a larger-diameterrear part, said hollow moving cylinder being stepped and having a radialshoulder between a smaller-diameter front part and a larger-diameterrear part, said annular piston being capable of sliding in leaktightfashion over said front part of said hollow moving cylinder and in saidrear part of said stepped bore; said annular piston, said rear part ofthe stepped bore, said radial shoulder and said front part of the hollowmoving cylinder together defining a variable-volume annular chamber,characterized in that said variable-volume annular chamber is capable ofcommunicating with an internal volume of said hollow moving cylinder viaat least one radial passage.
 8. The boosted braking device according toclaim 7, characterized in that a communication path between saidvariable-volume annular chamber add said internal volume of said hollowmoving cylinder is interrupted shortly after said hollow moving cylinderhas covered said axial distance.
 9. The boosted braking device accordingto claim 7, characterized in that a calibrated non-return valve islocated in a communication passage between said annular chamber and alow-pressure fluid reservoir.
 10. The boosted braking device accordingto claim 8, characterized in that a compression spring is located insaid annular chamber between said annular piston and said radialshoulder of said hollow moving cylinder.
 11. The boosted braking deviceaccording to claim 9, characterized in that, at rest, a first limit stopformed on said front part of said hollow moving cylinder keeps saidannular piston at said axial distance from said shoulder of the bore.12. The boosted braking device according to claim 11, characterized inthat, a second limit stop formed in the hollow moving cylinder issituated at the axial distance from a rear end of the secondaryhydraulic piston.